1. Field of the Invention
The invention relates to a process for the emulsion polymerization of conjugated dienes, particularly of butadiene. 2. Description of the Prior Art
In the emulsion polymerization of conjugated dienes, a heterogeneous system containing additives, such as the activator, the regulator, and other compounds, is formed from the aqueous phase containing the emulsifier and the monomeric hydrocarbon phase; cf. Ullmann, Enzyklopadie der technischen Chemie, Vol. 9, (1957), pp. 325-339.
One of the greatest disadvantages in the production of latices by emulsion polymerization of dienes is gel formation during the polymerization. In the case of butadiene this occurs at conversions of 10 to 80%, and is particularly pronounced from 25 to 60%. The gel formation is associated with a marked increase in the viscosity in the reaction medium, which presents a strong hindrance to stirring. Moreover, laminar layers of increasing thickness and low mobility are formed on the inside walls of the reactor; this is associated with a decrease in the heat-exchange coefficient in the reactor. Under these conditions the emulsion is no longer homogeneous, and difficulties arise in the control of the reaction temperature and the course of the reaction. Moreover, the latices obtained exhibit a pronounced scatter in their particle size. In less critical cases these phenomena can be at least partly eliminated by the use of anchor stirrers or scraper stirrers, which lead to renewal of the viscous layer in the vicinity of the reactor walls and hence improve the heat-exchange of the reactor. However, these measures has not led to success in the production of latices having high solids contents.
Another method that is used to prevent gel formation in the reaction system consists in the addition of considerable quantities of water at regular intervals during the polymerization to achieve substantial dilution of the thickened phase. However, this measure is not particularly advantageous, since the action of the water occurs only intermittently and the stirrability is not appreciably improved. Moreover, the product obtained has a low solids content, and its number average particle diameter is generally less than 1800 A and in any case less than 2000 A.
Another process consists in the addition, during the polymerization, of ionizable compounds whose action causes at least partial agglomeration of the particles, with the result that the surface area to be protected and hence the viscosity of the system decreases. However, this process is also of little advantage owing to the low reproducibility of the working conditions, the high ash content in the end product, and the pronounced lump formation.
Finally, another method makes use of mechanical agglomeration. However, this method is also associated with disadvantages, particularly because it is difficult to adhere precisely to the operating conditions and because particles with a broad particle size distribution are formed.